Support a user-provided string for the TLS ALPN extension.

webrtc/rtc_base/BUILD.gn

Index: webrtc/rtc_base/BUILD.gn
diff --git a/webrtc/rtc_base/BUILD.gn b/webrtc/rtc_base/BUILD.gn
index ee85a632ebc8cc1007ffb66e6a84aebbec1bac52..bbedc05486d36126b0b483e3962151d3edf89f7f 100644
--- a/webrtc/rtc_base/BUILD.gn
+++ b/webrtc/rtc_base/BUILD.gn
@@ -1036,6 +1036,7 @@ if (rtc_include_tests) {
     }
     if (is_posix) {
       sources += [
+        "openssladapter_unittest.cc",
         "ssladapter_unittest.cc",
         "sslidentity_unittest.cc",
         "sslstreamadapter_unittest.cc",
@@ -1054,6 +1055,14 @@ if (rtc_include_tests) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
+    if (build_with_chromium) {
+      include_dirs = [ "../../boringssl/src/include" ]
+    }
+    if (rtc_build_ssl) {
+      deps += [ "//third_party/boringssl" ]
+    } else {
+      configs += [ ":external_ssl_library" ]
+    }
   }
 }